Energy response simulation of 4π gamma ionization chambers using Monte Carlo technique

The energy response of 4pi gamma re-entrant well-pressurized ionization chambers was calculated using the MCNP4B Monte Carlo computer code. Two different chambers were simulated-one filled with argon and the other with nitrogen gases at 2 MPa pressure. The MCNP4B results were in good agreement with the experimental measurements and the EGS4 Monte Carlo theoretical results. Energy response formulas for both chambers were obtained with respect to the 1.173 MeV gamma energy line of Co-60. Detailed energy response in the low energy region was facilitated by proper curve fitting of the simulation results. The MCNP4B code was also used to examine the effect of the axial displacement of the standard gamma source in the chamber well. The highest chamber response occurs when the source is located at the center position of the well and it decreases with increasing distance from the center. The effect of the source axial position displacement on the chamber energy response is noticeable at medium and high-energy photon energies. (C) 2002 American Association of Physicists in Medicine.